Overview of strategy for eliminating the effect of snow from our polarization measurements: winter
- The raw polarization measurements made with SPARO (e.g. those currently posted to the web page under the Data Analysis
menu item) include contributions from the source but also from the instrument (SPARO and Viper). Part of this
instrumental polarization is constant and can be removed in a straightforward way, via observations of Sgr B2 and
the Moon. But snow on the mirrors can also cause instrumental polarization. We will refer to this contribution to
the instrumental polarization as snow polarization. Snow polarization is a serious problem. The way
that we dealt with this problem in winter 2000 was to install a camera to view the condenser and monitor its snow content.
Then we were able to exclude measurements that were taken when the condenser was sufficiently snowy to cause significant
polarization. This analysis is described in Dave Chuss' thesis. However, the
primary also tends to accumulate snow, and we have no camera on the primary. The assumption that we made in analyzing the
2000 data was that, because the primary is cleaned at the same time as the condenser, the amount of snow visible on the
condenser gives an accurate measure of the total amount of snow in the optical path.
- During winter 2003, Paolo has noted that often the condenser is clean but the primary is snowy. This may be
related to the very different observing elevation used in 2003 as compared with 2000. In any case, this
concern that we might be measuring fake polarization and not know it! Thus we need to come up with a strategy to deal with
this problem. The following section suggests one such strategy.
Preliminary ideas for dealing with snow polarization: winter 2003
- First note that the snow induced polarization has a characteristic angle of zero degrees, probably
related to the fact that the telescope is of the off-axis design. Because the condenser has a much
higher angle of incidence than the primary, the snow on the condenser will probably have a bigger effect.
This indicates that as long as we come up with some reasonable strategy for dealing with this effect,
we will probably be successful. Very drastic measures (installing new camera and lighting) are probably
not needed. we simply need an unambiguous way to ensure that we have eliminated the effect from our data.
- I suggest the following two-step approach:
- Paolo will come up with a system for quantifying the amount of snow on the primary mirror. E.g., zero
will be no snow at all, and higher numbers will indicate more snow. We will call this the primary mirror snow
An example of
such a system
is given in
Paolo examines the primary, he will record (1) the primary mirror snow number, (2) whether he cleaned the
primary or not, and (3) the time. These records will be used during analysis to determine a curve that gives
the snow polarization vs. primary mirror snow number. We will cut data as needed to ensure that we have
eliminated from our science results all traces of this effect.
- In order to make the curve described above, we need a measure of the snow polarizaiton. This can
be obtained by very frequently measuring a bright peak. When the polarization of this peak is affected by snow, it will
and this change will be clearly visible in the data if the source is bright enough. We will call this source
the snow monitor. For the upper
actuator position, rcw57(West) will be our snow monitor.
- Conclusion: if this approach is adopted, then from now on rcw57(West) will be observed every day, as
long as we are operating in the upper actuator position. If we
alternate between four polarimetry sky positions then we will be losing 25% of our
observing time on the snow monitor, but this is much better than having to
publish a paper in which we must qualify our conclusions by saying "the effects of snow may
have affected our results". As time goes on, we may be able to reduce the fraction of
the time that we spend observing our snow monitor. Especially if we find that
the effect is not severe.
Contact firstname.lastname@example.org. Last updated April 9, 2003.